1. Field of the Invention
The present invention relates to an image forming apparatus applicable to digital copiers, printer stations of facsimiles, digital printers, plotters and the like and more particularly, to an image forming apparatus adapted to directly form an image on a recording medium by projecting a developer thereto.
2. Description of the Related Art
As popular image forming methods of forming visible images on receiving media, such as sheet-like copy paper, based on electric signals supplied from computers, word processors, facsimiles and the like, there have generally been employed an ink jet system utilizing an ink, a heat transfer system transferring the visible image by fusing the ink, a system sublimating the ink and an electrophotographic system.
To cope with recent demands for a high speed image forming process and a high-quality image, there has been employed the electrophotographic system arranged such that a photoconductor is irradiated with light for forming a toner image thereon and the toner image is transferred to the recording medium such as a copy sheet. The image forming process includes the steps of converting an electric signal into light rays by way of a laser emitting device or an LED head; applying the light rays to the photoconductor preliminarily charged to a uniform potential thereby forming on the surface of the photoconductor an electrostatic latent image corresponding to intensities of the light rays; developing a toner image on the surface of the photoconductor by bringing toner into contact with or projecting the toner to the electrostatic latent image, the toner stored at a toner carrier; making the toner image electrically attracted to the recording medium; and subsequently applying both pressure and heat or either of them to the recording medium thereby to transfer the toner image to the recording medium. There exists a demand for a simplified structure of the electrophotographic image forming apparatus, which requires the photoconductor.
In this connection, there have been proposed image forming apparatuses which do not require the photoconductor and the like for forming an image on a recording member. For example, Japanese Unexamined Patent Publication JP-A 4-182151 (1992) has proposed an image forming apparatus which forms a toner image on a recording medium or a sheet in such a manner that the toner are charged by means of an electric field for electrically projecting the toner while a control electrode including a plurality of passage holes disposed in a toner-projection path is applied with a potential which is varied to make the toner attracted directly to the recording medium.
In image forming apparatuses represented by the apparatus disclosed in JP-A 4-182151 above-mentioned which is adapted to project the toner for forming an image directly on a copy sheet, the quality of a produced image greatly depends upon a construction of the control electrode, which controls the potential applied thereto thereby regulating a quantity of toner, as the developer, passing through the apertures thereof.
By maintaining constant distances between a control electrode member formed with the apertures and the control electrode, and a carrier carrying the toner thereon, and between the control electrode and a counter electrode, the toner projection is stabilized for recording images in favorable conditions. The prior-art control electrode member is small in thickness. That is, the control electrode member is formed of a flexible substrate which tends to be bent by an external force or other factors thus deteriorating the positional relations above-mentioned. This results in an unstable toner projection which adversely affects the quality of produced images.
Accordingly, the image forming apparatus disclosed in the patent publication above-mentioned utilizes a pair of bases fixed to places at opposite ends of the flexible substrate composing the control electrode member. Interposed between the bases is a spring for urging the bases in directions to increase a distance therebetween so that the control electrode may be held under a tension for prevention of the deformation of the control electrode member.
In the prior-art image forming apparatus such as disclosed in the patent publication, constant distances between the control electrode and the counter electrode and between the control electrode and the carrier are maintained by preventing the control electrode from being bent, whereby the toner projection is stabilized.
The control electrode member of the image forming apparatus is supported at a distance of about 100 .mu.m, for example, from the carrier. In such a prior-art construction, if warpage and undulation on the order of dozens micrometers is produced in the control electrode member or the flexible substrate of which the control electrode member is formed, the control electrode member mounted to a predetermined position may be deformed due to the warpage and undulation of the substrate. The control electrode is formed with multitudes of apertures for the passage of the toner, close to which apertures the control electrodes are formed, respectively. The apertures thus formed reduce the strength of the control electrode member so that a region formed with the apertures or the control electrodes is more susceptible to the warpage and undulation. The value of several dozens micrometers is quite great relative to the distance of 100 .mu.m. The dozens micrometers relative to 100 .mu.m account for quite a great error. In this case, a desired control of the toner projection is not always achieved. For example, if the distance between the control electrode member and the toner carrier is increased by dozens micrometers, an insufficient amount of toners is projected to form dots of an insufficient size or density, and a degraded image results. This leads to not only difficulty in producing a desired halftone but also to impossibility of favorable color reproduction by the color image forming apparatus.
Conversely, where the control electrode member deformed due to the warpage and undulation thereof defines a reduced distance to the toner carrier, an excessive amount of toner is projected to form dots of an excessive size or density. Naturally, this also results in a degraded image. In addition, the production of halftone becomes difficult while the color reproduction by the color image forming apparatus is adversely affected.
The production of warpage and undulation in the control electrode is unavoidable because a thermal processing is indispensable for the production thereof. In order to eliminate the warpage and undulation, the technique disclosed in the patent publication utilizes the spring to apply a tension to the control electrode.
In the arrangement wherein the control electrode member is subjected to the tension for elimination of the warpage and undulation therein, quite a great tension is required. Accordingly, a fear exists that the member for supporting the control electrode member cannot withstand the tension to be broken, or that the control electrode member cannot withstand the tension to suffer further production of warpage and undulation or to end up with the breakage thereof.